There has been technology relating to a sensor device which detects a predetermined physical quantity based on an instruction from a controller and transmits the detected value to the controller (refer to Patent Literature 1, for example). Such a sensor device communicates with a controller using digital signals, so that noise is generated due to signal changes. Radiation noise generated when a signal repeatedly changes its level between H (high) and L (low) adversely affects audio equipment and control devices used in surrounding areas.
A differential transmission method has been known as a method of high-speed transmission between a controller and a sensor device. In the differential transmission method, a transmission side and a reception side are connected by a pair of communication lines and information is transmitted between the two sides based on signal differences between the two communication lines. In the differential transmission method, when subjected to noise, the signals on the two communication lines are similarly affected, so that the contents of information being transmitted is not easily affected. This improves noise resistance (resistance to noise) of the communication. Regarding noise radiation from the communication lines, the controller and the sensor device are connected by a twisted pair cable, so that signal variations become symmetrical in the pair of cables. Hence, variations in magnetic flux caused by signal variations are canceled in the pair of cables and radiation noise generation is suppressed.
Noise generation from the sensor device itself, however, cannot be suppressed in the same manner as for the communication lines. Namely, in the sensor device, it is difficult to make circuit patterns symmetrical, so that suppression of radiation noise is difficult.
In this regard, according to the above-described existing technology, a plurality of circuit substrates are laminated through insulation layers, and the area of a conductive pattern on the H-level side of one of the circuit substrates is approximately equal to the total area over flat surfaces of a pair of circuit elements. In this way, charge variations due to voltage signal variations in a sensor device are reduced, and, radiation noise from the sensor device is reduced.
A radiation noise reduction method has also been known in which, in a sensor device using a printed circuit board, one of a plurality of circuit patterns is made as a ground pattern.